Generalized Hardcore Dimer Models approach to low-energy Heisenberg frustrated antiferromagnets: general properties and application to the kagome antiferromagnet

We propose a general non-perturbative scheme that quantitatively maps the low-energy sector of spin-1/2 frustrated Heisenberg antiferromagnets to effective Generalized Quantum Dimer Models. We develop the formal lattice independent frame and establish some important results on (i) the locality of the generated Hamiltonians (ii) how full resummations can be performed in this renormalization scheme. The method is then applied to the much debated kagome antiferromagnet for which a fully resummed effective Hamiltonian - shown to capture the essential properties and provide deep insights on the microscopic model [D. Poilblanc, M. Mambrini and D. Schwandt, arXiv:0912.0724] - is derived.Comment: 26 pages, 4 figures, EPAPS inlined, manuscript revised, corrected minor typos (notably figure 2)

Quantum Monte Carlo simulations of fidelity at magnetic quantum phase transitions

When a system undergoes a quantum phase transition, the ground-state wave-function shows a change of nature, which can be monitored using the fidelity concept. We introduce two Quantum Monte Carlo schemes that allow the computation of fidelity and its susceptibility for large interacting many-body systems. These methods are illustrated on a two-dimensional Heisenberg model, where fidelity estimators show marked behaviours at two successive quantum phase transitions. We also develop a scaling theory which relates the divergence of the fidelity susceptibility to the critical exponent of the correlation length. A good agreement is found with the numerical results.Comment: 4 pages, 3 figures; v2: added scaling theory; v3: published versio

Effective Quantum Dimer Model for the Kagome Heisenberg Antiferromagnet: Nearby Quantum Critical Point and Hidden Degeneracy

The low-energy singlet dynamics of the Quantum Heisenberg Antiferromagnet on the Kagome lattice is described by a quantitative Quantum Dimer Model. Using advanced numerical tools, the latter is shown to exhibit Valence Bond Crystal order with a large 36-site unit cell and hidden degeneracy between even and odd parities. Evidences are given that this groundstate lies in the vicinity of a $\mathbb{Z}_2$ dimer liquid region separated by a Quantum Critical Point. Implications regarding numerical analysis and experiments are discussed.Comment: 4 pages, 4 figures, deep revision of manuscript including new data, revised figures, new Fig. 2(c) and new reference

Valence bond distribution and correlation in bipartite Heisenberg antiferromagnets

Every singlet state of a quantum spin 1/2 system can be decomposed into a linear combination of valence bond basis states. The range of valence bonds within this linear combination as well as the correlations between them can reveal the nature of the singlet state, and are key ingredients in variational calculations. In this work, we study the bipartite valence bond distributions and their correlations within the ground state of the Heisenberg antiferromagnet on bipartite lattices. In terms of field theory, this problem can be mapped to correlation functions near a boundary. In dimension d >= 2, a non-linear sigma model analysis reveals that at long distances the probability distribution P(r) of valence bond lengths decays as |r|^(-d-1) and that valence bonds are uncorrelated. By a bosonization analysis, we also obtain P(r) proportional to |r|^(-d-1) in d=1 despite the different mechanism. On the other hand, we find that correlations between valence bonds are important even at large distances in d=1, in stark contrast to d >= 2. The analytical results are confirmed by high-precision quantum Monte Carlo simulations in d=1, 2, and 3. We develop a single-projection loop variant of the valence bond projection algorithm, which is well-designed to compute valence bond probabilities and for which we provide algorithmic details.Comment: 15 pages, 11 figures. Final version after minor revision

Approche liens de valence de la physique de basse énergie des systèmes antiferromagnétiques

L'objet de cette thèse est le traitement du modèle de Heisenberg antiferromagnétique dans la base de liens de valence, qui permet d'en décrire la physique de basse énergie. Le manuscrit est organisé en deux parties : dans la première nous utilisons le concept de fidélité afin de détecter les transitions de phases quantiques. Nous démontrons notamment que cette quantité est accessible dans un algorithme de Monte Carlo quantique, formulé dans la base de liens de valence, permettant ainsi de calculer la fidélité sur des systèmes de grande taille. La deuxième partie vise à développer l'idée initiale de Rokhsar et Kivelson, qui a pour but de transformer un modèle de Heisenberg en un modèle de dimères quantiques, généralement moins complexe d'un point de vue numérique. Après une dérivation rigoureuse, cette technique est appliquée au réseau kagomé et permet d'établir l'existence d'un point tricritique au voisinage du modèle initial. La même méthode est ensuite utilisée afin de traiter le modèle J1-J2-J3 sur le réseau hexagonal et démontre l'existence d'une phase plaquette dans un domaine de paramètres déterminé.The purpose of this thesis is the treatment of the antiferromagnetic Heisenberg model within the valence bond basis, allowing for the description of its low-energy physics. The manuscript is organized in two parts: In the first part we utilize the fidelity concept in order to detect quantum phase transitions. It is notably shown, that this quantity is accessible in a valence bond projector quantum Monte Carlo algorithm, making available the fidelity approach to large scale simulations. The second part is devoted to the generalization of an idea going back to Rokhsar and Kivelson, which aims to map a Heisenberg model onto a numerically less demanding quantum dimer model. Starting from a rigourous derivation, the method is then applied to the kagomé lattice and allows to establish the existence of a tricritical point in the vicinity of the original model. The same technique is also used to treat the J1-J2-J3 Heisenberg model on the honeycomb lattice, showing the existence of a plaquette phase in a determined parameter regime

Genome-wide association studies of the self-rating of effects of ethanol (SRE).

The level of response (LR) to alcohol as measured with the Self-Report of the Effects of Alcohol Retrospective Questionnaire (SRE) evaluates the number of standard drinks usually required for up to four effects. The need for a higher number of drinks for effects is genetically influenced and predicts higher risks for heavy drinking and alcohol problems. We conducted genome-wide association study (GWAS) in the African-American (COGA-AA, N = 1527 from 309 families) and European-American (COGA-EA, N = 4723 from 956 families) subsamples of the Collaborative Studies on the Genetics of Alcoholism (COGA) for two SRE scores: SRE-T (average of first five times of drinking, the period of heaviest drinking, and the most recent 3 months of consumption) and SRE-5 (the first five times of drinking). We then meta-analyzed the two COGA subsamples (COGA-AA + EA). Both SRE-T and SRE-5 were modestly heritable (h2 : 21%-31%) and genetically correlated with alcohol dependence (AD) and DSM-IV AD criterion count (rg : 0.35-0.76). Genome-wide significant associations were observed (SRE-T: chromosomes 6, rs140154945, COGA-EA P = 3.30E-08 and 11, rs10647170, COGA-AA+EA P = 3.53E-09; SRE-5: chromosome13, rs4770359, COGA-AA P = 2.92E-08). Chromosome 11 was replicated in an EA dataset from the National Institute on Alcohol Abuse and Alcoholism intramural program. In silico functional analyses and RNA expression analyses suggest that the chromosome 6 locus is an eQTL for KIF25. Polygenic risk scores derived using the COGA SRE-T and SRE-5 GWAS predicted 0.47% to 2.48% of variances in AD and DSM-IV AD criterion count in independent datasets. This study highlights the genetic contribution of alcohol response phenotypes to the etiology of alcohol use disorders

Evaluating porous polylactide-co-glycolide/bioactive glass composite microsphere powders for laser sintering of scaffolds

While laser sintering (LS) processes are typically optimised towards denser powder beds, our approach is different as we investigate the use of non-dense powder beds made of porous composite powder materials of polylactide-co-glycolide (PLGA) and bioactive glass (BG) 45S5. Powders were produced via solid-in-oil-in-water emulsion with modified BG as porogen and changing PLGA:BG ratios with up to 50 wt% of BG. Characterisation was done using scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FTIR), X-ray fluorescence (XRF), thermal gravimetric analysis (TG), powder packing and flow, particle-size distribution (PSD) and X-ray tomography. The microparticles showed varying degrees of sphericity, porosity and specific surface area (SSA), while the BG porogen could be incorporated into or onto the foamed polymer. Powders were used for LS and gave parts with an overall porosity of ~ 75%. Our special approach might be interesting for multi-material LS for real biomimetic scaffolds with tailored hierarchical pore structures

Alcohol use disorder is associated with DNA methylation-based shortening of telomere length and regulated by TESPA1:implications for aging

Chronic heavy alcohol consumption is associated with increased mortality and morbidity and often leads to premature aging; however, the mechanisms of alcohol-associated cellular aging are not well understood. In this study, we used DNA methylation derived telomere length (DNAmTL) as a novel approach to investigate the role of alcohol use on the aging process. DNAmTL was estimated by 140 cytosine phosphate guanines (CpG) sites in 372 individuals with alcohol use disorder (AUD) and 243 healthy controls (HC) and assessed using various endophenotypes and clinical biomarkers. Validation in an independent sample of DNAmTL on alcohol consumption was performed (N = 4219). Exploratory genome-wide association studies (GWAS) on DNAmTL were also performed to identify genetic variants contributing to DNAmTL shortening. Top GWAS findings were analyzed using in-silico expression quantitative trait loci analyses and related to structural MRI hippocampus volumes of individuals with AUD. DNAmTL was 0.11-kilobases shorter per year in AUD compared to HC after adjustment for age, sex, race, and blood cell composition (p = 4.0 × 10(−12)). This association was partially attenuated but remained significant after additionally adjusting for BMI, and smoking status (0.06 kilobases shorter per year, p = 0.002). DNAmTL shortening was strongly associated with chronic heavy alcohol use (ps < 0.001), elevated gamma-glutamyl transferase (GGT), and aspartate aminotransferase (AST) (ps < 0.004). Comparison of DNAmTL with PCR-based methods of assessing TL revealed positive correlations (R = 0.3, p = 2.2 × 10(−5)), highlighting the accuracy of DNAmTL as a biomarker. The GWAS meta-analysis identified a single nucleotide polymorphism (SNP), rs4374022 and 18 imputed ones in Thymocyte Expressed, Positive Selection Associated 1(TESPA1), at the genome-wide level (p = 3.75 × 10(−8)). The allele C of rs4374022 was associated with DNAmTL shortening, lower hippocampus volume (p < 0.01), and decreased mRNA expression in hippocampus tissue (p = 0.04). Our study demonstrates DNAmTL-related aging acceleration in AUD and suggests a functional role for TESPA1 in regulating DNAmTL length, possibly via the immune system with subsequent biological effects on brain regions negatively affected by alcohol and implicated in aging